Secret signaling



April 14, 1925. 1,533,311

` H. FLETCHER -SECRET SIGNALING Filed Deo. 1s; 1923 OTO / (5f. /6/ g? D8550;" my;

@y Mfr/Wy Patented 'Api'. 1 4, 1925.

HARVEY FLETCHER, or NEW YoRx, N. Y., AssIGNo-R To wEs'rERNnLEcrRm com:F

PANY, INcoRroRATED, or NEW YoRx, N. Y., A coRPoRA'rIo'N QFNEW YORK.

sEoRnr s Application led VlDecemyher To all who/m, it my concern."

Be it known that I, HARVEY FLETCHER, a citizen of the United States ofAmerica, re-

siding at New AYork city, inl the county of- 5 New York, State of NewYork, have invented certain new and useful Improvements in SecretSignaling, of which the follow- -ing is a full,I clear, concise, andexact descri tion.

lo T e present invention relates to wave Y transmission with secrecy. f

The invention is applicable to the art 0f signaling and to thetransmission of electrical waves for any purpose.

15 An object of the invention is to secure a high degree of secrecy in asignaling o r other` wave transmission s stem while at the same timemaintaining til the system and the qualityf the reproduced waves.

It has been proposed to divide the waves that are to be transmitted intosubbands of frequency components and to render the waves diiicult ofrecognition by transposing certain of the sub-bands of components prior'to the transmission of the wave.

' Such transposition may comprise interchanging two or nore of thesub-bands as to the frequency positions normally occupied bythe-respective sub-bands, or it may comprise inversion of the frequencyorder-0f the components ofone or more of the sub-bands either with orwithout an accompanying interchange being made between the frequencypositions of the'sub-bands. Still other transformations for the waveshave been pro- From the standpoint of--f'adaptability of a secrecysystem tostandard transmission cir- 40 cuits, it is desirable that thealtered waves -whichlare actually transmitted occupy the same absolutefrequency range as -the waves would occup if' left in .their normal orunalteredcondltion From the standpoint of secrecy, it is desirablethattransformations made in the waves. be such as will produce ymaximumconfusion when -an attempt is made'to receive them on any ordinar andunauthorized receiver. From the stan point of economy in transmitted`power it is desirable that no noise components be added and that no'components other than those derived from the waves themselves beV sent.

e general '-eiiiciency of IGNALING.

1s, 192s. Serial'No. 681,346.

The invention comprises, among its' features, provisions for securingeach /of the above-mentioned desirable results.

vThe various objects' and features of the invention will more fullyappear from the description now to be given of the embodinientillustrated in the accompanying drawmff. 4 ln this drawing, Fig. 1 showsin symbolic form the manner of (lvision of the normal speech intosub-bands a, b and c and the mam nerof tansposing the sub-bands prior totransmission according lto the invention.-

Figs. 2 and 3 show, in schematic orm,la

transmitting system and a-receiving system,

respectively, for carrying 'onsecret signaling in accordance with theinvention..y v

It has been found that the intelligibility of a sub-band of frequenciesdivided out of y the speech band depends not only on the breadth of thesub-band but also upon the region of the speechl band from which thesub-band is taken.

It is also found 'that the energy sented in any sub-band of speech ofgiven frequency width diers markedly depending upon the region of thespeech bandfrom -which it is taken. The degree of intelligibility of asub-band does not as arule correspond at all closely with the energy4which itl possesses, so that it is possible yteg-divide 1- fnormalspeech into sub-bands of the same frequency width and same energycontent but of widelydifferent degrees of-intelligi bility, and,further, sub-bands of different frequency width and the sameintelligibility may'diifer widely in energy content.

sii,

As "might be expected from what has justv been said ,relative totheeffects of venergy content andfrequency position of sub-ban it has beenfound that the' amount of cony fusion that may be produced by'asub-bandof noisecurrents of givenfre' and energy content, differs. widey..accord1ng as the noise-components are introduced into one region oranotherof the normal speech range. Smce. any selected` sub-band ofspeech can be converted into nolse'or cono5 uency with v fusioncomponents bymaking some trans'-- position of the components, either byinverting theirorder within the sub-band or -by shifting. `the'-mfrequency position of the los` sub-band', 'the 'total@olif-1181.011"pro'ducedlfroml j so operating on a sub-band dependsmarkedly upon the width of sub-band that is selected,'the re ion of thenormal speech band from whic it is taken and the nature of thetransposition made of it.

For a fuller' discussion of the relations between 'band width, enercontent and intelligibility of portions o the speech band, reference maybe had to applicants paper on The Nature of Speech and itsInterpretation, published in Journal of the Frank"y y llin Institute fordune 1922, Vol. 193, No. 6.

- In dividing the speech band into subbands for secret transmission inaccordance with the invention, advantage is taken of the dependence ofthe degree of intelligibility upon the frequency width, the fre uencyp0- sition, and the energy content of t e various sub-bands that may bederived from normal speech. Also, in making the required transformationsin the transmitted waves in order to produce confusion, 'the inventionband c in spiteof its 'greater fleqluencysub-band' Zrmay extend from 800to1'600 cycles and the sub-band c from 1600 to 3000 cycles. On thisbasis, the a sub-band has anintellligibility of about 16%, 'the bsublband `27% and the c sub-band 25% while the voice energy representedby the asubband is aJbofut72%, that of the b sub-band is 18% and that ofthe c sub-band is 6%. Each sub-band by itself is seen to be highlyunintelligible since it enables only one sound in four, or one sound insix in the case of the a sub-band,to be understood. With this scheme ofsub-division, it will be seen that the same sub-band width hasgreateriintelligibility-in the b position than in the'a position andthat the intelligibility in the sub'- width is lonly of the .same orderas at of the sub-band` Ib. It will also be seen that the su -band al ofthe least intelli ibility has the eatest energy content an that the smal.energy handle although this sub-'band possesses the greatest frequencywidth.

Sub-bands a and b being of .equal freuency width readily permit of beinginterc anged in their; ,frequency position. The three possi-ble.transformations on this basis of sub-divisionfare shown oppositetherev- 'spective number slhxin Fig'. f1. The 'com-, V blnation'4 changingthe the mod ulatin will be not Von y to reproduce the sub-band contentis that of the subfrequency position of the sub-bands a and b. Thesecond combination is obtained by shifting the sub-band b to the aposition and by shifting the sub-band a to the b position and invertingits frequency order as is shown by the reversal of the arrow. The thirdcombination is obtained by shifting the subband a to,the'b position andshifting the sub-band b to the a., position and inverting its frequencyorder.

If an attempt is made to receive any of' the combinations 1, 2 or- 3 inan ordinary telephone receiver the intelli'bility will be` practicallynil. lt is-foun that merely 'shifting a sub-band of the speechfrequencies to a higher or lower frequency position without disturbingthe frequency order within the sub-'band renders the sub-band highlyunintelligible. In each of the three combinations therefore, about 90%of the received sound energy comprises confusion or noise components sothat the small intelligibility of the sub-band a is completelyoverwhelmed by these noise components.

If an attempt is made to break the secrecy of these combinations byusing a l heterodyne oscillator, thefreuency of which may be variedthroughout t e range of the received frequencies the best than can beaccomplished will Ibe the restoration of one of the sub-bands to itsnormal position in the s h range butthis restored sub-band will iieecber ofy confusion or noise components.A For example, let it be su posedthat a 'heterodyne oscillator' is a )usted to receive the sub-'band a ofthe first combination by havin itsirequency set at 800 cycles. If the su-bands of combination 1 are used to modulate a' carrier -wave theheterodyne oscillator'would, of course, vhave-its frequency avesuperposed upon it a large numl made equal to the correspondingcomponent -in the side band, that isf-thecomponent produced from the800-cycle,l component in wave. The effect of this a in its normalposition 'in the lspeech range but also to superpose-thersubfband b upothe sub-band a and to displace the'sub-band.

c downward from its normal vposition by 800 cycles. Both t e sub-band band the sub-band etherefo a pear as noise currents. Further, the su bandb cannot -be separated from the sub-band a by `filteringy since thevoccupy the same frequenc ran If-theyheterodyne oscillator be seyt toceive the sub-band b of combination 1 prop# erly, both the sub-bands aand c are displaced upward in the frequency scale 800 cycles from theircombination position and will, therefore, appear as noise currents. Onaccount of the greater energy content of v-the sub-band a the noiselproduced b itwill be much greater than the volume'o the restoredsub-band b.

the restored sub-hand..

It has been assumed above that the scheme of transposition Vofthetransmitted wave is In practice, how-ever, these conditions would-Vrarely, lf'ever., 'exist and the chances of obwhich are niet ibilitylaccurate tuningwith an intelligit blhtg so low is Very small.

:und 3 which illustrate one form of carryin rent derived from 3'O phone10 are transmitted throu h the reknown to the unauthorized-.observer andthat he is able to locate the frequency` of the heterodyn'e oscillatorexactly at the desired point in the various frequency combinations.

taining even a`s good intelligbilit as is assumed above; would be verysmal, first, because there will alwaysvbe other distortions l consideredin the above dis cussion, 'and secondly, because the proba- Referencewill 'now be made to Figs. 2 g out t invention. In Fig-2 the speech curlthe circuit of the micropeating coil 11 to the three filters 1, F2 andF3 which serve to divide the speech currents into three sub-bands a,band c as indicated of 800 to 1600 cycles and 40 in Fig. 1. The filterFris designedl to transmit selectively the compoxents 0 to- .800\cycles" and to ei'ectiyely suppress components of lall otherfrequencies. 'Similarly,{ the lilterl2 transmits `only the componentsthe filter F3 transmitsonly components of 1600 to3000, cycles as wellasthe other filters employed inthe system.. Each of the@ filters may beof the type disclosed'in U." S.' patent to Campbell N0. 1,227,113, May22, 1917. .i

The output Vof. the filter F,l is passed through a repeating coil 12 andis applied differentially to the grids of the two tubes of the 'balancedmodulator Mx. A' source of waves ,13 is applied from thegrids of thesetubes in parallel through the 'transformer 14. The resulting modulationcomponents are `transmitted vthrou h the output transformer 15 totheoutgomglter F4. The fil.` ter F4 is designed'to .have a transmissionrange of 800 to 1600 cycles and to suppress the transmission through itof components of all otherfrequencies. The frequency'of ythe sourse 13depends-upon the lparticular frequency transformation that is to be produced inthe sub-band a. If this sub-band is to be shifted as in.combination l 1 of. Fig. lso that it occupi/es the subfba-nd' bposltlon, source 13 willl have a frequehcyofSOO cycles.

-The modulation com'ponentswill comprise a'n upper and a lower side`band,.the upper side band extending from 800 cycles to 1600 cycles. Thisside band will beilselectively transmitted by the filter F4 while theother side band will be completely ysu pressed." l

Since the transmission ranges of t e filters F1 and F4 do not overlapthere will be no direct transmission through the 'modulating system ofany i unchanged. A The modulator circuit M1 is eral type disclosed in'U.S. l.patent to Carson No; 1,343,306, June 15,1920. i

The output of the filter F2 isfimpressed..

f thc genl l.

of the a sub-band frequencies on a second balanced modulator'M., which,

may be of the same type as themo'dulatory M1 but which, for simplicity,in the drawing, is merely indicated by a rectangle. This modulator issupplied from a 'source 16, with a wave, the frequency of lwhich dependsupon the 'particular transformation that is to be made inA the outgoingwave'.Y If the b sub-band is tobe shifted to the wsub-'band position asin combination 1 ofFig. '1, the

source 16 will have a frequency off800 cycles.

As a result of the action ofthe"modulator circuit M2., side bands willbe produced,I one frequency between the applied b sub-band and thefrequency displaced downward in frequency by 800 400 of which is'represented by the 'difference in of the source'16, that is',-

-each component of the b ysub-band will bev cycles so that the resultingside b'andwill l occupy the frequency range, 0 to S00 cycles. This sideband will' be selectively trans-V mitted through the filter F5 to theexclusion of components -of all otherifrequencies.l Since no shift is tobe made in the frequencies of the sub-band c, no modulator 1s required,but the filter F,l has its' output connected in common with the outputsof filters F4 and F6 to the common repeatin coil 17.

If desired thesub-bands from` t 1e. filters F-3,F4 andFs'could betransmitteddirectly over a telephone line or other circuit to thedistant 'receiving station. These sub-bands would in 'such case appearmerely asunin-i telligihle noise4 -in any ordinary 'receiver 'whichmight be associated with the hne slnce4 they .representa normal speechwave trans- A formed in accordance withcoinbination-l of Fig.'- 1, theeffect of: which has been 'discussed above. i

The particular type of transmission cn',- cuit that has been chosen forillustration, however, isa .radio transmission system, so

'that the transformed speech wave is used in the system illustrated tomodulate aradio f requencywave which ma;` belderived from' Ianysuitable'oscillator 1 8. The'r'atlio waves are a plied in parallel tothe` grids of the modu ator M and the secret speechwaves are vapplieddili'erentially to the modulatorgrids. As a result, the unmodulatedcarrier I component from source 18 is suppressed and only the two sidebends of the secret s eech based on the radio wave are pro uced.

`aide bands are impressed on the transmittingantenna TA by means of thetrans- 1- former19. The antenna TA may be tuned to i transmitselectivelyone 'of the two side fbandsito -the exclusion of the other. Or, in

:case the radio; frequency is t'oo high to perl mit of-discriminatien1between the side bands.

extending from 800 to 1600 cycles. -Due to `the, .characteristics ofthel-iiltcr F'4 only the 2,0-lowe'r side band is transmitted. Since thisside. band represents: the ditlerence'- in fre- .quency'rhetweentheappliedfl-'wavesthe fre- JAquen'cy order within'this'side ban'd'isl thereverse of that ofthe applied a sub-band.

,To'obtain the necessary shiftofthe subband b for vcombination 2, thesource 16 is made have a frequencyof 800 cycles as '1n 'thecase ofcombination' 1.".

e source 16 is. made to have. a frequency l yof 160Q cycles. `-Inthis'case an upper side band, extending' from 2400't'o 3400 cycles is i.obtained and a'lower side band extending 4latter of vthese two. sidelbands* is' transmitted through the filter. F:s to thefoutgoing cwcuiuxThe Wave trzinsmitted from the station. of

IRv which betunedto the frequency of 'the wavestolbe received. .Thewaves picked 'uP by4 thlantenna RA are transmitted-into y limpressed on.the detectopD lthe.-inpu{; 'of

'the tuned circuitQO 'from which they .are

. which is also supplied with a wave of the radio orcarrienfrequencyfrom source 21.

i f As a-'rcsult off-.the interactionin the detest'- o ing circuit Aofthe received-Smetana ,with the locally, supplied carrier' wave, a 'bandthrough the filter F3', u

"so impressed on the receiver represent the .To obtain ,combination of'Fig. l, the 39 sourcel is made to have a frequency of 800 `golesas.1n"the. case of com ination 1 and'.

therefore, the speech which was to be transv Fi 2 is received on thereceiving antennav What is claimed is: lof frequency. components isproduced ofthe same character, as those used to modulate' the carrierrat. -th1 a` transmitting. station.y components are ,divided into sub--bandsjby-the filters Fl, Fz-nnd-GF," iwhic'li-4 may'duplicatesrespectively of the filters sub-band l1selectivelyz -p''assed by-l-thef.representsthe transposed sub-.band thejqspeechfwaves, and transmittingsaid 17e-125 lmlpessedfon/theinpt offthe des f' `as the source 16ofF-ig. 2. As a result of the action of the demodulator circuit DMl, twoside bands are produced, the upper ofv which extends from 800to 1600cycles and represents. the restored sub-.band b. This, sub-band' isselectivelypassed bythe filter FJ to' the exclusion of'componentS, oflall other frequencies. Thel sub-band from'the 'filterFz represents thetransposed sub-band "a and is impressed on fthe` demodulator 4DMz whichmay be ofthe same type as" the demodulator DM1 and which is suppliedwith a wavel from source 13 of the same frequency-as source 13 of Fig,2; The relsultinglower side bamd'occupies the fre-v quency range. 0 vto800 cycles and 1s selec,-l tively passed by the filter` F4 to theexclusion of components ofjall other frequencles. j The'remalningsub-band representlng the unchanged sub-band c is transmitted direct-"which may be a plicate'of the filter F3, ancl is impressed incommonwith the waves from; the filters F4 and F5 vupon the receiver R.''llhe waves 910 restored sub-bands originally derived` from the speechcurrents transmitted from".the repeating coil 11 of Fig. 2 andrepresenL'S mitted,

While it is preferable, for the sake of economy in transmitting energyand 1n the frequency range employed where transmissiontakes place ybymodulating a ,carrier or 'radio wave to suppress the c rrier and totransmit'only one side'band tiis is not essential' to the invention-L Itwill be understood from what .is stated above that any' type oftransmission, either carrier or radio transmission or directtransmission over a line at 'speech `frequencies ma be' employed. i

ile it hasl been assumed above that speech issub-divided into -onlythree subbands, two of'which are of the' same fre- -quency width, vit iswithin the invention to employ more than two sub-bands of the samefrequency and to interchange them prior to transmission together withone or more sub-,bands which are unchanged.l 115 1. The method ofsecretly 'transmitting speech waves comprising dividing the waves -intosub-bands of frequencies, the sub-bands differing among themselves asIto the breadth 4120 of included fre uencies, interchanging cersul'tantwaves to a'distance.-l

thod of secret 'transmission' of 2, The

l"speech comprising dividing the normal speech' components v, into'sub-bands including a plurality of 'culi-bands 'of the same fre-1,311'

- quency width `and a sub-band comprising .the higher frequency regionof normal having smaller voice energy, interchanging the frequencyposition Aof certain of said sub-bands with -others while keeping theresultant n'aves Within 'practically the same speech waves, andtransmitting the resultant Waves.

4. 'lhe method of secret transmission of speech which comprises dividingnormal speech waves into a sub-band of frequency componentspossessingsmall voice energy f and low intelligibility, and into othersubbands possessing low intelligibility but greater voice energy thanthe first-mentioned sub-band, transposing only the subbands of largerenergy contentamong themselves to reduce the intellgibility of theresultant waves below that of any sub-band by Jitselfl, and transmittingthe Wave so transpose n Y 5. The method of secret transmission of speechcomprising transmitting certain of the frequency components of normalspeech in their normal frequency relations, selecting sub-bands of otherfrequency compon- `cnts from normal speech and interchanging thefrequency positions of the sub-bands to produce unintelligible confusioncomponents occupying as a whole the same' fre; quency positions asbefore their transposition, and transmitting the interchanged componentsalong with the unchanged components. Y f n:

6. The vmethod of .secret telephony coml prising dividinv normal speechwaves into fi uency components, eachsub-bands of sub-band by itseossessing small intelligibility, said subands differing among themselvesin energy contentv and in frequency` width, producing noise currents romcertain of the sub-bands of compon len ts'of the? larger energycontent'f'ndtransnnttmg the resulting noise 'currents'along `with theunchanged' components -whereby the intellgibility of the wave componentsas a whole is practically nil, the" transmitted divide normal speecextreme frequency lnnits as the original,

Wave occupying substantially the same frequency limits as the originalspeech.

Y 7 In a telephone system an incoming circuit, an outgoing circuit, asource of speech Waves in saidincoming circuit, means includingfilters'for dividing the speech waves into sub-bands of frequencycomponents,

means between .said circuits foitransmitting certain frequencycomponents from said source directly to the outgoing circuitsubstantially unchanged 4in frequency, means for interchangng thefrequency positions of certain sub-bands of components from said sourcewhile keeping them as a whole content andy each by itself'possessingsmall intelligibility, means to transmit substantially unchanged asub-bandxof relativelyl small energy content and means to interchangethe frequency, positions of sub-bands of relatively large energy contentand to transmit .the interchangedv sub-bands along wvith the unchangedsub-band.

9. In a secret telephone system, means to modulate a high frequencycarrier wave in accordance with the unchangedand interchanged sub-bandsproduced by the..

means defined in claim 8, means for sup- .pressing one of the side-bandsof the resulting modulated vvavev and means for transmitting theremaining'sidefban'd.

10. The method of receiving and rendering intelligible the wavestransmitted by the method defined in claim 1 comprlslng dlviding thereceived waves into res ective subbands composing the wave'that 1stransmitted, and inter'changing in reverse manner the sub-bands thatresult from interchanging sub-bands of normal speech, to restore thespeech. 11. A receivingsystem forreceiving and renderingintelligible'the waves transmitted by a .system as vclail'ned incla-im7,compr1s 4ing filters for dividing the received waves into sub-bandsofvfrequency components, a 'receiver, means to transmit the componentsfromone of said lilters directly to said receiver, and means forrestoring to normal frequenc position the sub-bands from others o `saidfilters and for impressing the restored sub-bands on said receiver.

In witness whereof, I hereunto subscribe my name this 12 day of DecemberA. D.,

A.1923. HARVEY FLE'rCHIizR.

